U.S. Pat. No. 4,863,655, inventors Lacourse et al., issued Sep. 5, 1989, describes the disposal problems associated with most presently used packaging materials formed from synthetic polymers. For example, although expanded polystyrene is a resilient, compressible and low density (about 0.25 lb/ft.sup.3) protective packaging filler material and performs its protective function well (e.g. as the ubiquitous "peanuts"), it is not biodegradable. Thus, U.S. Pat. No. 4,863,655 also describes a biodegradable packaging material as an alternative to expanded polystyrene comprising an expanded amylose starch product. Although biodegradable, the special high amylose component used is quite expensive. Also, another difficulty with the solution described by U.S. Pat. No. 4,863,655, is that the expanded amylose starch material is not, by itself, suitable for preparing containers where moisture resistance is a necessary property (e.g. various take-out food containers).
Starch behavior during extrusion and injection molding has been the focus of considerable studies. The state of starch in these various physical changes has been described under names such as melted starch, molecularly dispersed or disrupted starch, destructured starch, and so forth. A historical review of starch transformation when extruded is described by Shogren et al., "Development of Starch-Based Plastics--A Reexamination of Selected Polymer Systems in Historical Perspective," Starch/Starke, 45, pp. 276-280 (1993).
U.S. Pat. No. 5,095,054, inventors Lay et al., issued Mar. 10, 1992, describes blends of destructurized starch and one or more polymers. Although these blends and articles made therefrom are said to show an improved dimensional stability in humid air when compared with non-blended destructurized starch, nevertheless the articles are said to retain a high degree of disintegration in contact with liquid water. Although this disintegration may assist in disposal of such articles in landfills and the like, the high degree of disintegration in contact with liquid water is disadvantageous for many applications where substantial moisture is present.
U.S. Pat. No. 5,272,181, inventors Boehmer et al., issued Dec. 21, 1993, discusses starch-graft copolymers as another packaging material alternative, which materials are also said to readily disintegrate under wet conditions. Although such disintegration may assist in integrating the materials into soil, the disclosed materials are not entirely biodegradable due to the presence of the synthetic resins (such as polyacrylate). Further, as already mentioned, such a wettability property is disadvantageous for applications where moisture will be present.
U.S. Pat. No. 5,185,382, inventors Neumann et al., issued Feb. 9, 1993, describes biodegradable packagings formed from starch and a polyalkylene glycol or derivative. A preferred polyalkylene glycol is polyethylene glycol. However, use of these water soluble components mean that expanded products have little resistance to moisture. Further, the bulk density property from many Of the formulations tends to be rather high.
Accordingly, attempts to find expanded compositions that are biodegradable, reasonably competitive in price with commodity plastics such as polyethylene or polystyrene, but which are more environmentally friendly and which have adequate moisture resistant properties, continue to be made.